Turbines for aircraft engines as well as gas and steam powered turbines for industrial applications comprise at least one rotor carrying multiple stages of airfoils (henceforth referred to as blades), which rotate with respect to the turbine case. In turn, a turbine case carries multiple stages of airfoils (henceforth referred to as guide vanes), such that the turbine consists of alternating stages of blades and guide vanes. To reduce leakage of air, steam, or combustion gas over the tips of the blades, shrouds can be disposed on the radially inner surfaces of the stator so as to form a ring seal around the blade tips. To limit air, steam, combustion gas, or the like from entering the rotor cavities or wheel spaces, blades have platforms and guides vanes have inner sidewalls. Similarly, guide vanes have outer sidewalls to limit radially outward leakage. Together, the blades, guide vanes and shrouds define the primary flowpath inside the turbine.
During use, the turbine components can experience degradation. Currently, periodic inspections, such as borescope inspections, are performed in order to assess the condition of the turbine in-between service intervals. Examples of damage that can be observed during inspection include wear (e.g., from incursion of blade tips into the shrouds, particle-induced erosion, water droplet induced erosion, wear due to sliding contact between stationary components), impact (e.g., spallation of thermal barrier coating (TBC) or environmental barrier coating (EBC) from turbine-section components, leading edge burring/bending of compressor blades), cracking (e.g., thermal fatigue, low-cycle fatigue, high-cycle fatigue, creep rupture), edge-of-contact damage between stationary parts, oxidation or hot corrosion of high-temperature metallic sections, static seal degradation, and creep deformation (e.g., of guide vane sidewalls/airfoils, blade platforms, and tip shrouds).
During service intervals, the turbines are at least partially disassembled to allow repair and/or replacement of damaged components. Currently, damaged components of turbine engines are primarily repaired at overhaul or component repair facilities, with only limited intervention conducted in the field. The processes used to repair compressor and turbine flowpath components may include surface cleaning so as to remove accumulated dirt and oxidation products, stripping and restoration of coated surfaces, crack repair, section replacement, aero contouring and smoothing, and the like. Although the cost of the repair operations is typically lower than the component replacement cost, waiting for planned outages may run some fraction of components beyond their repair limits and may even lead to unplanned outages caused by the failure of the most heavily distressed component. Consequently, conventional systems and methods of maintaining turbines using periodic inspections and factory-based component repair methods may not be optimal.
To address the above described problems, there have been disclosed in situ methods of repairing a damaged component, such as an abradable coating (e.g., U.S. Pat. Nos. 7,509,735, 8,563,080, U.S. Patent Application Publication No. 2015/0174837). However, these prior disclosures are focused on borescope-type access using small-diameter, tethered tools. These tools are severely limited for the purpose of maintenance because they are constrained in size and can access only limited locations within the gas turbine engine. Furthermore, even if a damaged area can be reached, the orientation of the tool to a damaged area may be disadvantageous for performance of an inspection or repair task. The size restrictions imposed by borescope access port diameter limit the capability of inspection and repair tools, and the access path may cause jamming or severing of the tether. Accordingly, the present invention seeks to provide a novel system of maintaining turbine assemblies that overcomes the above-mentioned limitations by using a remotely controlled maintenance apparatus that is inserted in the machine via alternate routes, such as axially through the turbine, and may be operated with or without a tether.